During the formation of a semiconductor memory cell (e.g., during a metal-1 etch and other etches), word lines tend to acquire high voltage (i.e., the word lines tend to charge up), while the substrate remains at a much lower voltage. This difference in voltage causes Fowler-Nordheim (FN) tunneling to occur through the gate stack and causes the charges to be trapped in the charge storage area of the memory cell. As a result, the initial threshold voltage (Vt) of the memory cell can change. This change in the initial threshold voltage of the memory cell is commonly referred to as in-line process charging, which is a highly undesirable phenomenon. In addition, this charging process also damages the tunnel oxide of the memory cell and causes reliability issues.
Typically, a charging protection structure is connected to the memory arrays to limit the charging effect during the process. A charging protection structure may be a diode structure, or a fuse structure or other structures that would short the memory array to ground once the potential of the word lines of the memory array exceeds a certain voltage level, thus preventing the word lines from charging up to an undesirably high level. The charging protection structure is typically connected to the core array only after metal-1 formation. However, process charging can occur before the metal-1 layer is formed. Therefore, there is a need for a charging protection structure that can protect the memory cells before the metal-1 layer is formed.